I talked with Matlab's Technical Support, and between us we found the solution.
First Matlab uses a file known as pathdef.m to define its paths. This file is located at %MATLAB_ROOT%\toolbox\local. I was using a startup.m file to add the path to my *.mexw64 file, but this extra path was not updating the pathdef.m file that Matlab uses. Apparently, (even though I was starting Matlab as a user with administrator rights), the pathdef.m file was not getting updated by startup.m. The solution was to manually edit pathdef.m, save it to a temporary folder, and then to copy it to the %MATLAB_ROOT%\toolbox\local location. (Windows 7 asks perimission to perform the copy when you do this). After doing this Matlab can now see ConnSpiderServer.mexw64 (my mex command).
